1. Field of the Invention
The invention concerns a method to operate an image-generating medical modality (in particular a magnetic resonance tomograph) and a corresponding image-generating medical modality.
2. Description of the Prior Art
A magnetic resonance tomography (MRT) apparatus frequently has a tube-like stationary unit with which the electromagnetic fields required for an imaging method are generated during operation. Such a tube-like stationary unit has a tunnel-like and centrally arranged opening or recess in which the generated electromagnetic fields act during operation, and in which a subject to be examined (a patient, for example) is positioned for an examination by means of imaging methods.
This tunnel-like recess is bounded by parts of the surface of the stationary unit that are designated herein as an inner tunnel wall. Located directly behind the inner tunnel wall are some of the assemblies of the stationary unit with which the electromagnetic fields are generated. When the spatial distribution of the time-dependent electromagnetic fields within the recess is considered, it appears that stronger alternating electromagnetic fields are active in close proximity to the inner tunnel wall of a magnetic resonance tomography apparatus than in the remaining regions of the recess. Therefore, in this immediate close proximity to the inner tunnel wall, there is an increased risk of so-called radio-frequency burns (RF burns) at which a tissue damage of a patient occurs due to injected radio-frequency radiation.
In the clinical field of MRT systems, patients sometimes come very close to the inner tunnel wall or even come into contact therewith, either due to the physical dimensions (for example corpulent and/or adipose patients) or due to asymmetrical positioning (for example in the examination of the elbow). The increased risk of radio-frequency burns must therefore be taken into consideration. Compliance with the recommended global SAR (Specific Absorption Rate) limits for the safe operation of cylindrical RF transmission coils (which are classified as volume coils in the IEC standard (60601-2-33) responsible for the safe operation of MRT systems) no longer safely protects against local RF burns wherein clearance is too small.
In order to avoid RF burns, two procedures have previously been established. The first is to significantly reduce the global SAR limits of the cited IEC standard (or possibly other limit regulations—FDA, for example) so that, for all operating conditions, no RF burn occurs even in the case of direct contact with the inner tunnel wall. However, this produces a marked reduction of the performance and, for example, an extension of the examination time within which the RF pulses are radiated, but also compromises with regard to the quality of the generated images (for example the use of sub-optimal flip angles or RF pulse shapes). In order to avoid the reduction of the performance from being too drastic, current MRT systems are often equipped with a contact protection function. The aforementioned limitation is thereby adjusted individually for the given situation (patient, figure, size, position relative to the RF transmission coil) by an assessment of individual measurement variables determining the local load (for example local E-fields, amperages in the conducting structures of the transmission antenna). Both variants frequently assume the cooperation and responsibility of the user (operator/technician) in that the user is enjoined to make sure that the patient has a minimum clearance (in mm) from the inner tunnel wall. This then enables the limitation of the performance to not be quite so drastically dimensioned. However, it is disadvantageous that the risk of an RF burn due to an incorrect operation of the system (here the positioning of the patient by the user) is increased. In the large majority of measurement situations, however, the clearance from the inner tunnel wall is greater than the critical distance x, which is typically ≦20 mm. The performance is therefore very often unnecessarily limited.
Alternatively or additionally, it is sought to produce a reduction or attenuation of the electromagnetic fields locally in close proximity to the inner tunnel wall by structural and technical measures, but this approach has an impermissibly high cost associated therewith.
From US 2003/0098688 A1 a method is known in which the position of the patient relative to the transmission antenna is determined exactly by an imaging magnetic resonance pre-measurement before the implementation of the actual measurement (thus the diagnostic examination of a patient by means of magnetic resonance imaging). On the basis of this exact position of the patient, the SAR (Specific Absorption Rate) values are then calculated in a known manner for planned parameters of the measurement from known patient data and the exactly determined position of the patient relative to the transmission antenna. The parameters are modified as necessary until the SAR values lie within the limit values, and the actual measurement for magnetic resonance imaging is subsequently implemented.